It depends how you define their importance. But some people here don't read every comic-book that was released by Marvel.

People who haven't read the comic-books are more familiar with Quicksilver/Scarlet Witch being part of the X-Men world simply because of the videogames that were released and the cartoons that were aired in TV. And they were also depicted as Magneto's children in those videogames and cartoons.

This is true, but I'm just saying in terms of the actual source material the twins are not that important to X-Men or Magneto and definitely not essential, especially Wanda. You can make the case Pietro has a place because of his involvement with X-Force, but that's it.